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Unit 1 anchorage systems

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anchorage devices

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Unit 1 anchorage systems

  2. 2.  Pre tensioning system  Post tensioning system
  3. 3.  The various types of devices used for tensioning steel are grouped under four principal catagories. ◦ Mechanical ◦ Hydraulic ◦ Electrical or Thermal ◦ Chemical
  4. 4. Mechanical devices Hydraulic devices  It includes weights with or without lever transmission.  Geared transmission in conjunction with pulley blocks.  Screw jacks with or without gear drives.  In simple it is wire binding machines.  Hydraulic jacks are the simplest means of producing large prestressed forces or extensively used as tensioning devices.  This systems are used for the ranges of 5 to 100tonnes  Large hydraulic jacks for forces in the range of 200 to 600tonnes have also been developed by baur-leonhardt.
  5. 5. Electrical or Thermal devices Chemical devices  It have been used successfully in 1958 for tensioning of steel wires and deformed bars.  The steel wires are electrically heated and anchored before placing of concrete in the moulds.  It is also called as thermo- electric prestressing.  Expanding cements are used and the degree of expansion is controlled by varying curing conditions.  Since the expansive action of cement by sitting in restrained it induce tensile forces in tendon and compressive forces in concrete.
  6. 6.  Freyssinet anchorage system  Gifford Udall systems  Lee-McCall systems  Magnel blaton system  BBRV system  Baur Leonhardt system
  7. 7.  This system is widely used in Europe and India  It consists of a cylinder with a conical interior through which the high tensile wires pass and against the walls of which the wedged by a conical plug lined longitudinally with grooves to house the wires.  The main advantage of this system is that a large number of wires or strands can be simultaneously tensioned using double acting hydraulic jacks.
  8. 8.  It is developed in UK.  It consists of steel split cone and cylindrical female cone anchorages to house the high tensile wires bearing against steel plates.  Each wire is tensioned separately and anchored by forcing a sleeve wedge into a cylindrical grip resting against a bearing plate.  The ducts are generally formed by metal sheaths cast into the concrete member.
  9. 9.  In this method, the tendons comprise high tensile bars of diameter varying from 12 to 40mm which are threaded at the ends.  After tensioning, each bar is anchored by screwing a nut and washer tightly against the end plates.  In this system the forces are transmitted by the bearing at the end blocks.  While the system eliminates the loss of stress due to anchorage slip, it has a disadvantage in that curved tendons cannot be used.
  10. 10.  This system adopts metallic sandwich plates, flat wedges, and a distribution plate for anchoring the wires.  Each sandwich plate can house up to four pairs of wires.  The distribution plate may be cast into the member at the desired location.  The number of wires in the magnel cable varies from 2 to 64.
  11. 11.  This system is well suited for transmitting large forces.  A BBRV tendon consists of several parallel lengths of high tensile wires, with each end terminating in cold formed button head with a machined anchorage fixture.  In the case of tendons formed by strands, they are anchored to the machined fixture by split cone sleeves.